Re: [PATCH] drm/i915/dp: Fix link retrain loop on pre-gen8 platforms with DP adapters

["Сергей Мавленков" <s.mavlenkov@mail.ru>]

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Hi Jani,

Thank you for the pointer. I've filed the bug here:
https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/15994
The full dmesg, hardware details, root cause analysis, and proposed fix
are all in the issue.

Best regards,
Кусь

> 
> Четверг, 30 апреля 2026, 10:45 +03:00 от Jani Nikula <jani.nikula@linux.intel.com
> >:
> On Thu, 30 Apr 2026, bitey <s.mavlenkov@mail.ru> wrote:
> > On Ivy Bridge (and other pre-gen8 platforms), active DP-to-HDMI protocol
> 
> > converters such as the Chrontel CH73111 trigger an infinite link retrain
> 
> > loop on kernels 6.x+. The monitor connected through the adapter goes
> > black and recovers approximately once per second, eventually causing
> > system instability.
> >
> > Three changes introduced in the i915 DP stack combine to cause this:
> >
> > 1. intel_dp_stop_link_train() now automatically schedules a
> post-training
> > link check via intel_encoder_link_check_queue_work(). The Chrontel
> > adapter reports non-standard DPCD link status, which causes the check
> > to fail and trigger a retrain, looping indefinitely.
> >
> > 2. intel_dp_needs_link_retrain() unconditionally returns true when
> > seq_train_failures > 0, bypassing the actual DPCD link status read.
> > A single marginal training attempt permanently marks the link as
> > needing retrain.
> >
> > 3. intel_dp_short_pulse() forces LINK_STATUS_CHANGED on every short HPD
> > for all platforms, causing unnecessary link checks that initiate the
> > retrain cycle on adapters that generate frequent short HPDs.
> >
> > Fix all three by gating the new behavior behind DISPLAY_VER(display) >=
> 8,
> > restoring the 5.15-era behavior for Ivy Bridge/Sandy Bridge and earlier.
> 
> > These platforms use the older g4x DP path and do not benefit from the
> > aggressive post-training link monitoring.
> >
> > Tested on Intel HD 2500 (Ivy Bridge, device ID 0152) with a Chrontel
> > CH73111 DP-to-HDMI adapter driving a 1920x1080 display over 2 lanes
> > at HBR (270 MHz).
> 
> Please file a bug over at [1] with the full dmesg reproducing the
> issues, and reply with the link to the bug.
> 
> Thanks,
> Jani.
> 
> 
> [1] https://drm.pages.freedesktop.org/intel-docs/how-to-file-i915-bugs.html
> 
> 
> 
> >
> > Signed-off-by: bitey <s.mavlenkov@mail.ru>
> > ---
> > drivers/gpu/drm/i915/display/intel_dp.c | 8 +++++++-
> > drivers/gpu/drm/i915/display/intel_dp_link_training.c | 3 ++-
> > 2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c
> b/drivers/gpu/drm/i915/display/intel_dp.c
> > index ORIGINAL..FIXED 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -5676,6 +5676,7 @@ static int
> > intel_dp_read_link_status(struct intel_dp *intel_dp,
> > u8 link_status[DP_LINK_STATUS_SIZE])
> > {
> > + struct intel_display *display = to_intel_display(intel_dp);
> > int err;
> >
> > memset(link_status, 0, DP_LINK_STATUS_SIZE);
> > @@ -5712,7 +5713,8 @@ static bool intel_dp_needs_link_retrain(struct
> intel_dp *intel_dp)
> > if (intel_dp->link.retrain_disabled)
> > return false;
> >
> > - if (intel_dp->link.seq_train_failures)
> > + if (DISPLAY_VER(display) >= 8 &&
> > + intel_dp->link.seq_train_failures)
> > return true;
> >
> > /* Retrain if link not ok */
> > @@ -5965,6 +5967,7 @@ static bool
> > intel_dp_short_pulse(struct intel_dp *intel_dp)
> > {
> > + struct intel_display *display = to_intel_display(intel_dp);
> > bool reprobe_needed = false;
> > u8 esi[4] = {};
> >
> > @@ -5988,7 +5991,8 @@ intel_dp_short_pulse(struct intel_dp *intel_dp)
> > * TODO: let the link status check depend on LINK_STATUS_CHANGED
> > * or intel_dp->link.force_retrain for DPCD_REV >= 1.2
> > */
> > - esi[3] |= LINK_STATUS_CHANGED;
> > + if (DISPLAY_VER(display) >= 8)
> > + esi[3] |= LINK_STATUS_CHANGED;
> > if (intel_dp_handle_link_service_irq(intel_dp, esi[3]))
> > reprobe_needed = true;
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > index ORIGINAL..FIXED 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > @@ -1154,7 +1154,8 @@ void intel_dp_stop_link_train(struct intel_dp
> *intel_dp,
> >
> > intel_hpd_unblock(encoder);
> >
> > - if (!display->hotplug.ignore_long_hpd &&
> > + if (DISPLAY_VER(display) >= 8 &&
> > + !display->hotplug.ignore_long_hpd &&
> > intel_dp->link.seq_train_failures < MAX_SEQ_TRAIN_FAILURES) {
> > int delay_ms = intel_dp->link.seq_train_failures ? 0 : 2000;
> 
> --
> Jani Nikula, Intel
>

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